Circadian rhythms and fatigue: a discrimination of their effects on performance.

Different effects between circadian rhythms and fatigue have been discriminated. The discrimination was based on human performance changes. It was shown that such changes, not related to loss of sleep, are more easily attributable to either circadian rhythm or fatigue effects. However, when they follow loss of sleep, discrimination becomes difficult. Consequently, a new, more direct way of describing them and their cause has been formulated. This type of change is then related to the jet-lag-by-time-zone myth.